CNET On Cars: Car Tech 101: Octane demystified
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CNET On Cars: Car Tech 101: Octane demystified

5:28 /

Brian Cooley tells you the secrets of octane: what it is, what it does, and what happens when you're cheap.

[MUSIC] Now we fill up on gasoline in dribs and drabs over the course of a year. So the price difference between Premium and Regular may seem small on a pervasive basis. [MUSIC] Yes, the percentage difference between the two grades has declined due to the more rapidly increasing cost of a gallon of any kind of gas. But the real cost spread of premium over regular has increased over the last six years in the US. Put another way it used to cost you a little under $150 extra bucks extra a year to run premium, assuming 15,000 miles a year and 20 MPGs in your car. But around 2004 it began to take off and now it's a choice that cost more like $240 a year, real money. Let's find out if it's worth it. [MUSIC] Now, premium gas has this image, largely due to marketing, as being somehow maybe a more explosive kind of fuel that blows off all this additional power. In fact, it's kinda the opposite, premium gas is high octane. Gasoline is, is first a blend of many components that have many different properties. And as such octane is one of those properties. High octane allows you to compress the fuel more, higher air pressure, higher heat without spontaneous combustion. If you use regular or low octane gas in an engine that wants higher, the mixture may combust spontaneously in the cylinder before the spark sets it off. That's called Pre-etonation or Knock. That's the sound of your engine slowly coming apart. It's hurting your engine. There's a little mini explosion that goes on in a 4-cycle engine. One of the cycles is compression. When that goes out of whack, there's an explosion that's not good. It's a physical pressure wave travelling in the combustion chamber and you're actually hearing the result of that pressure wave knocking against the metal parts of the combustion chamber. These explosions, they can hurt the internal of the engine. They can hurt the catalytic converter of the engine. The science that goes on when the four cycle engine does this combustion process- It's pretty high tech. When one of those elements is out of whack, it means it's not happening right The high compression engines were all the rage back in the muscle car and pony car era, of course. But lately car makers have been getting into high compression for everyday cars because they get more power out of little tiny engines that need less air and fuel to fill the cylinder each cycle. In other words, they create good power and use less gas. [MUSIC] Now why does premium gas or high octane gas cost more? This is a very contentious area. Refiners will tell you they get less of it out of a barrel of crude than they do regular. Secondly they add different additives to it that will be used to increase the octane, and those are more expensive than just crude itself. The main additive back in the day used to be tetraethyl lead which increases the octane beautifully except it's lead we don't do that anymore. But that's what gave rise to the old nickname for premium, they used to call it Ethyl. With the removal of lead, you remove an octane source, and so other sources of octane were required. Now, today's modern cars, as you probably know, have sensors and computers and variable components all over them. As a result, they can sense knock from running low octane gas, and adjust the engine slightly to compensate for it, knock it out. But there's a limit to that adaptability, depending on your car's design. That's why you wanna just check your car manual. It'll often tell you what octane level is okay and another one that's recommended. Also maybe one to avoid on the minimum side. Sometimes regular is fine. Other cars say mid grade at least, premium at best. Others say premium only. Okay now when you go to the gas pump, you can look at the actual number button that you're gonna select and look at the fine print and it will talk about a minimum octane number and under that it will typically say this sort of bit of math, R+ M over 2. That's research octane plus motor octane, divided by two, or the average of the two. So they're single cylinder engines that are used to calculate the octane of a given gasoline. So the research number tests kind of got lower engine speeds, so it tends to be higher, and then the motor octane is conducted at higher engine speeds, and tends to be lower. We take the average of the two, and you come up with what we call anti-knock index. And that's likely the number you'll see in a modern car manual. U.S. automakers have recently groused that octane in the U.S. is so low, they can't make the kind of engines they offer in other markets, smaller ones with even more power and greater efficiency. This one point of clarification is that in places like Europe and South America where the octane numbers that people call for in the high 90s 97, 98 that's only the research number right? Here in the US we do the research plus the motor and divide that by two. [MUSIC] Put it in a nut shell you're safe using the lowest grade of gas that your car manual doesn't forbid. That's your baseline. Typically, especially in higher performance cars, though, there are tangible benefits to using the ultimate grades of premium, especially if you want the full performance you paid for [MUSIC]

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